High enthalpy tunnels are for example used to simulate the extreme conditions which occur when a spacecraft enters the atmosphere of the earth. Because of the typically great velocity of the spacecraft, a strong pressure shock develops in front of the spacecraft. The strong pressure shock results in an impinging flow which is at least partially converted into heat. To prevent the spacecraft from burning during re-entry into the atmosphere of the earth and to keep the spacecraft maneuverable, it is very important to examine the influence of strong pressure shocks on flying bodies. In a high enthalpy tunnel, such a strong pressure shock is produced. However, the strong pressure shock in a high enthalpy tunnel only lasts for a very short period of time. Typically, after a set up period of 1 to 2 Milliseconds (ms). a stationary flow is developed within the high enthalpy tunnel. This stationary flow is the actual flow which is used for an experiment. The stationary flow only lasts up to 1 ms. Therefore, it is clear that a determination of the parameters of the flow, which is a gas flow, is quite complicated, especially, if it is intended to determine the parameters of the gas flow defined in time during the experiment period.
A method for determining at least one parameter of a gas flow, the method being based on atomic absorption spectroscopy, was first used in 1993 in the laboratory of Prof. Hanson (Stanford University) for temperature measurements behind a reflected shock in a small shock tube. To apply the method on an essentially cold flow in a high enthalpy tunnel, atoms or molecules of a specific resonance frequency and energy state need to be available in the gas or plasma flow in sufficient number.